In an assembly line of a machine assembly factory, there are many cases in which parts are attached and assembled by fastening screws, and an assembly process includes many screw fastening processes. In many screw fastening processes, an automatic screw feeder is used in order to reduce a process time of fastening screws.
Generally, an automatic screw feeder has a hopper for storing many screws and a conveyance part for taking screws out of the hopper and conveying the screws to a screw pick-up part (a position where the screws are picked up by a screw driver) while aligning the screws. Screws stored in the hopper move automatically to an inlet of the conveyance part due to a vibration of the hopper, and the screws enter the conveyance part one after another with the same orientation. The screws entering the conveyance part move along a screw passage of the conveyance part due to gravity or vibration, and are carried to the screw pick-up part. The screws moved to the screw pick-up part are attracted by a bit of a screw driver and taken out of the screw pick-up part. In the above-mentioned automatic screw feeder, there is a problem often occurs in that a screw feeding operation stops due to screw jamming at the hopper or the inlet of the conveyance part. The screw jamming is caused by a variation in the shape of screws or a variation in the orientation of the screws.
Although there is suggested a mechanism to scoop screws in a hopper using a swing arm and supply the screws to a conveyance part, it is difficult to align the screws scooped by the swing arm because the orientations of the screws scooped by the swing arm are all different and the screws tend to cling with each other. Thus, there also is a problem that a screw feeding operation stops due to screw jamming.
Thus, a method is used in which screws are accommodated in holes arranged in a plate-like tray and the screws are picked up by a screw driver one by one. Specifically, many screws are put in the holes of the plate-like tray in a matrix arrangement and the tray is located at a pick-up position of the screws by a screw driver. This method is used in many cases as a screw supply method for supplying screws, such as, for example, a screw provided with an O-ring, that are difficult to handle by a hopper and a conveyance part.
However, in order to pick up the screws from the plate-like tray one by one, an operator must locate a driver bit to a different position each time because a position at which each screw is picked up differs from screw to screw, which is inconvenient for the operator. Additionally, there is no good method which can locate screws to the holes of the plate-like tray. Thus, the screws are located to the holes of the plate-like tray manually, which causes an increase in a manufacturing cost.
Thus, there is suggested use of a cassette for accommodating screws instead of a plate-like tray, which cassette accommodating screws along a line in the same orientation with screw heads facing above (for example, refer to Patent Documents 1 and 2).    Patent Document 1: Japanese Laid-Open Patent Application No. 8-192371    Patent Document 2: Japanese Utility Model Registration No. 3041627
The cassette, which accommodates screws by being arranged along a line, has a slot that can hold screw heads. Accordingly, the screws are accommodated in the slot of the cassette in a state where the screws are arranged along an extending line of the slot and the screw heads of the screws are supported by elongated members forming the slot.
In a conventional single line accommodation cassette, in order to accommodate threaded parts of screws in a slot while supporting the screw heads, the width of the slot is slightly smaller than the diameter of the screw heads and slightly larger than the diameter of the threaded parts. Accordingly, when feeding screws having a different diameter, the cassette must be replaced with another cassette suitable for the screws having the different diameter, which is inconvenient for the operator.
Moreover, there still is no good method of loading screws to the cassette. A screw loading machine for aligning and feeding screws using a vibration or a swing arm has a complex structure and expensive.
Thus, it is desired to develop a simple structure screw feeder having a single line accommodating cassette or magazine that can be used with different diameter screws. Additionally, it is desired to develop a screw loading apparatus having a simple structure, which can load screws to a single line accommodating cassette or magazine.
In a conventional screw feeder, screws are attracted by a magnetized bit of an electric screw driver to perform a screw fastening operation (disclosed in, for example, Japanese Laid-Open Patent Application No. 4-5306). If the screws are made of a non-magnetic material, the magnetized bit is not effective and a suction type electric screw driver is used to attract each screw to a driver bit by suctioning. However, it is difficult to pick up the screws from the screw feeder by suctioning each screw at a screw pick-up position.